Project Summary Hirschsprung's disease (HD) is a developmental disorder of the gastrointestinal tract in which the distal enteric nervous system is absent, leaving that segment aganglionic. The most serious complication of HD is Hirschsprung's-associated enterocolitis (HAEC), an inflammatory colitis that causes distension, diarrhea, and fever. It occurs in up to one-third of children with HD and can lead to bacterial translocation, sepsis, and death. Although abnormalities in epithelial barrier function, mucosal immunity, and the microbiome have been proposed in the etiology of HAEC, the role of these factors is unknown. Lack of this knowledge hinders development of effective treatments to prevent HAEC onset. The objective of this proposal is to determine the contribution of the intestinal microbiome to the development of HAEC. Based on our published preliminary data showing an early and sustained disruption of the microbiome in a HD mouse model, we hypothesize that an aberrant microbiota is necessary and sufficient for the development of HAEC. The rationale for this study is to provide a sound basis for future testing of targeted probiotic and antimicrobial therapies for HAEC prevention in clinical trials. Our expertise includes microbiome analysis, clinical care of children with HD, use of HD mouse models, and bacterial transfer experiments into germ-free mice, creating a strong collaboration with the necessary skills to achieve the stated goals. Aim 1 is designed to determine whether the aberrant microbiota associated with HAEC in mice is colitogenic. Fecal transfer experiments will be performed from mice with HAEC to wild-type germ-free mice, and to conventionally raised EdnrB-/- mice prior to HAEC onset. Microbiota composition and function will be monitored in recipient mice, and they will be assessed for clinical and histologic evidence of enterocolitis. In Aim 2 we will characterize the microbiome associated with human HAEC and determine its colitogenic potential. Longitudinal changes in microbiome structure and function will be determined in feces collected from patients with HD during episodes of HAEC and remission. The colitogenic potential of human HAEC-associated microbiomes will be established by transferring HAEC microbiota into germ-free mice. Combining thorough characterization of fecal microbiome structure and function with fecal transfer experiments represents an innovative approach to defining the colitogenic potential of HAEC- associated microbiota in both mice and humans. The proposed experiments will yield important new knowledge that could have significant impact on our understanding and treatment of HAEC and yield generalizable knowledge, experimental approaches, and potential therapies for this and other intestinal inflammatory diseases. The proposed project will also expose students to basic and translational research, and strengthen the research environment at the University of Wyoming. !